Browsing by Author "Lee, SI"

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Comparative analysis of specific heat of YNi2B2C using nodal and two-gap models
(2006-01-01)
Huang, CL
;
Lin, JY
;
Sun, CP
;
Lee, TK
;
Kim, JD
The magnetic field dependence of low temperature specific heat in YNi2B2C was measured and analyzed using various pairing order parameters. At a zero magnetic field, the two-gap model, which has been successfully applied to MgB2 and the point-node model, appear to describe the superconducting gap function of YNi2B2C better than other models based on the isotropic s-wave, the d-wave line nodes, or the s+g wave. The two energy gaps, Delta(L)=2.67 meV and Delta(S)=1.19 meV, are obtained. The observed nonlinear field dependence of the electronic specific heat coefficient, gamma(H)similar to H-0.47, is quantitatively close to the gamma(H)similar to H-0.5 expected for nodal superconductivity or that can be qualitatively explained using a two-gap scenario. Furthermore, the positive curvature in H-c2(T) near T-c is qualitatively similar to that in the other two-gap superconductor MgB2.
X-ray absorption and optical spectroscopy studies of (Mg1-xAlx)B-2
(2003-09-01)
Yang, HD
;
Liu, HL
;
Lin, JY
;
Kuo, MX
;
Ho, PL
X-ray absorption spectroscopy and optical reflectance measurements have been carried out to elucidate the evolution of the electronic structure in (Mg1-xAlx) B-2 for x=0.0, 0.1, 0.2, 0.3, and 0.4. The important role of B 2psigma hole states to superconductivity has been identified, and the decrease in the hole carrier number is quantitatively determined. The rate of the decrease in the hole concentration agrees well with the theoretical calculations. On the other hand, while the evolution of the electronic structure is gradual through the doping range, T-c suppression is most significant at x=0.4. These results suggest that the superstructure in (Mg1-xAlx)B-2, in addition to the sigma holes, can affect the lattice dynamics and contributes to the T-c suppression effect. Other possible explanations like the topological change of the sigma band Fermi surface are also discussed.